1. Field of Invention
This invention relates generally to structural health monitoring and non-destructive evaluation, and more specifically to spiral-shaped arrays used as effective ways to direct energy without the need for individual control of the array components.
2. Background and Related Art
Structural health monitoring (SHM) is essential to prevent failure in a number of aerospace components, civil infrastructures, and mechanical systems. Research in this field is playing a key role in improving safety and reducing maintenance costs. The use of guided waves (GWs) has recently attracted great interest because of their potential to perform long-range and fast inspection, in contrast with conventionally employed normal beam ultrasonic scanning, which is local, and hence time-consuming. However, effective interrogation of remote areas of a structure using GWs requires suitable techniques for a correct localization of damages or anomalous acoustic sources such as impacts or leaks.
Multiple transducers are typically employed for this purpose, which are often arranged in array configurations. This allows for beam steering at selected directions, but comes at the cost of significant hardware complexity needed to individually control large numbers of elements, and/or intensive signal processing required for the implementation of processes such as delay-and-sum. Drastic hardware simplification and cost reduction of GW-based SHM could be achieved by using transducers with inherent directional capabilities. This involves designing the transducer shape or the arrangement of the sensing material so that it exhibits preferential radiation/sensing directions. Examples of devices with inherent beam steering capabilities include the CLoVER transducer as well as rosette configurations of macro-fiber composite (MFC) devices.
Guided wave methods have been proposed for SHM of plate-like structures using permanently attached piezoelectric transducers, which generate and sense waves to evaluate the presence of damage. Effective interrogation of structural health is often facilitated by sensors and actuators with the ability to perform directional scanning. This enhances the sensitivity of the inspection and simplifies the determination of the damage location.
Wave steering through phased arrays is a well-established technique, used extensively in ultrasonic imaging for medical and nondestructive evaluation (NDE) applications. Recent research has investigated the application of guided wave-based phased arrays for SHM. One conventional method implements a phased comb transducer array using hardware and software delay-and-sum beamforming algorithms on pipes.
In another, a virtual beam steering concept named EUSR using permanently attached lead zirconate titanate (PZT) transducer arrays. Another presents a circular array integrated with a deconvolution algorithm to improve imaging quality. Others have explored the optimization of linear arrays and presented a two-dimensional (2D) square array capable of virtual beam steering in a 360° range. Others have compared phased-array beamforming results using monolithic PZT and MFC transducers and concluded the latter have better directivity at certain steering angles.
Still other researchers propose the use of spatially distributed arrays, comprising sensors distributed over a large area, as an effective approach to image damage inside and outside the area enclosed by the array. In this regard, algorithms have been proposed that apply delay-and-sum procedures to each pixel of the image, so that waves can be considered as steered to each pixel point.
All these promising results not only demonstrate the potential benefits of beam steering for guided waves generation and sensing, but also underline some limitations. One such limitation is inherent in the array basic principle of operation, i.e., the delay-and-sum algorithm, which requires wiring and multiplexing of individual array elements.
In fact, most ultrasonic phased array technologies require electronic beam steering devices and corresponding hardware complexity, which makes their implementation as embedded devices problematic. For this reason, there is growing interest in the development of sensors and actuators with inherent directional and beam steering properties, so that cost and hardware limitations of traditional phased arrays can be partially overcome.
One such attempt is guided wave steering by firing in sequence multiple sections of wedge-shaped anisotropic piezo-composite transducers arranged in a circular ring pattern. Another configuration includes a two-dimensional (2D) periodic array of piezoelectric discs featuring frequency-dependent directivity in virtue of the interference phenomena associated with the spatial arrangement of the array elements. All elements of the array are fired simultaneously, so that the control hardware is reduced to a single channel, and beam steering is achieved through a sweep of the excitation frequency. In addition, the spatial arrangement of the array components is such that radiation is achieved at specific wavenumbers, so that it can be tuned to a specific wave mode.
To overcome the operability, efficiency and cost issues mentioned above, an improved structural health monitoring and non-destructive evaluation system is highly desirable. It is the intention of the present invention to provide for such industrial needs.